Restore extensions in core (#114921)

* bring back grammar extensions

* fix hygiene

* Revert "pull themes from the marketplace"

This reverts commit 0738f76dac.

extensions/theme-seti/package.json

@@ -0,0 +1,23 @@
+{
+	"name": "vscode-theme-seti",
+	"private": true,
+	"version": "1.0.0",
+	"displayName": "%displayName%",
+	"description": "%description%",
+	"publisher": "vscode",
+	"license": "MIT",
+	"icon": "icons/seti-circular-128x128.png",
+	"scripts": {
+		"update": "node ./build/update-icon-theme.js"
+	},
+	"engines": { "vscode": "*" },
+	"contributes": {
+		"iconThemes": [
+			{
+				"id": "vs-seti",
+				"label": "%themeLabel%",
+				"path": "./icons/vs-seti-icon-theme.json"
+			}
+		]
+	}
+}